Implement changes to plugin manifest processing and interfaces

[str-anger]

1. [x] Support default values in variable definition
2. [x] Support bool type (checkbox)
3. [x] multiline -> textarea
4. [x] Dropdown strings (select -> option?)
5. [x] Save log into an experiment; return a file name
6. [x] Expose a single experiment for a function
7. [x] implement display_name field for parameters